The rebanim have given very clear guide lines in connection to this topic.
1- the boy/girl is not obligated to tell the shadchan or the boy/girl in advance. Perhaps the shidduch will not work out anyway, and he/she does not have to compromise his privacy.
2- After they have met a couple of times and it seems to be progressing, but before it gets so serious as to cause heartache, he /she is obligated to tell her.
Also, please remember: The same pasuk that says “Al taleich rechil b’amesecha (don’t speak loshen hocccc
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